Climatologists and forecasters say next week's release of 2018 temperature data might show it was New Zealand's hottest on record.
Prof Jim Salinger of the University of Otago told NZME his calculations put the mean temperature on land at 13.5degC - 0.85degC above the average recorded between 1981 and 2010.
That would put it 0.1degC hotter than 2016, New Zealand's current record-holder.
"January, March, July and December were all at least 1degC above normal," he told NZME, adding that January was a whopping 3.2degC hotter than normal - New Zealand's hottest month on record.
Sea surface temperatures were also well above normal, with November's 3degC hotter than usual.
Prof Salinger said the oceans around New Zealand were 0.8degC above average overall in 2018, and at the other end of the scale, there was a record loss of ice from the Southern Alps - 9 percent in one year.
- How NZ must transform to combat climate change
- Humans 'don't have 10 years' left thanks to climate change - scientist
Chris Brandolino of NIWA predicted 2018's figures would track closely to, if not exceed, 2016's record temperatures.
NIWA said on Twitter 2018 it was "very warm", but insisted it hasn't yet been confirmed as the warmest.
"The final figure will be released on Tuesday... at NIWA's 2018 Annual Climate Summary conference in Auckland."
2017's average temperature was 13.15degC - the fifth-warmest on record, behind 2016, 2013, 1999 and 1998.